The present invention relates to high-performance permanent magnet materials based on rare earth elements and iron, which make no use of Co that is rare and expensive.
Magnetic materials and permanent magnets are one of the important electric and electronic materials applied in an extensive range from various electrical appliances for domestic use to peripheral terminal devices of large-scaled computers. In view of recent needs for miniaturization and high efficiency of electric and electronic equipment, there has been an increasing demand for upgrading of permanent magnets and in general magnetic materials.
Now, referring to the permanent magnets, typical permanent magnet materials currently in use are alnico, hard ferrite and rare earth-cobalt magnets. With a recent unstable supply of cobalt, there has been a decreasing demand for alnico magnets containing 20-30 wt % of cobalt. Instead, inexpensive hard ferrite containing iron oxides as the main component has showed up as major magnet materials. Rare earth-cobalt magnets are very expensive, since they contain 50 -65 wt % of cobalt and make use of Sm that is not much found in rare earth ores. However, such magnets have often been used primarily for miniaturized magnetic circuits of high added value, because they are by much superior to other magnets in magnetic properties.
In order to make it possible to inexpensively and abundantly use high-performance magnets such as rare earth-cobalt magnets in wider fields, it is required that one does not substantially rely upon expensive cobalt, and uses mainly as rare earth metals light rare earth elements such as neodymium and praseodymium which occur abundantly in ores.
In an effort to obtain permanent magnets as an alternative to such rare earth-cobalt magnets, studies have first been made of binary compounds based on rare earth elements and iron.
Existing compounds based on rare earth elements and iron are limited in number and kind compared with the compounds based on rare earth elements and cobalt, and are generally low in Curie temperature (point). For that reason, any attempts have resulted in failure to obtain magnets from the compounds based on rare earth elements and iron by casting or powder metallurgical technique used for the preparation of magnets from the compounds based on rare earth elements and cobalt.
A. E. Clark discovered that sputtered amorphous TbFe.sub.2 had a coercive force, Hc, of as high as 30 kOe at 4.2.degree. K., and showed Hc of 3.4 kOe and a maximum energy product, (BH)max, of 7 MGOe at room temperature upon heat-treating at 300 to 350.degree. C. (Appl. Phys. Lett. 23(11), 1973, 642-645).
J. J. Croat et al have reported that Hc of 7.5 kOe is obtained with the melt-quenched ribbons of NdFe and PrFe wherein light rare earth elements Nd and Pr are used. However, such ribbons show Br of 5 kG or below and (BH)max of barely 3-4 MGOe (Appl. Phys. Lett. 37, 1980, 1096; J. Appl. Phys. 53, (3) 1982, 2404-2406).
Thus, two manners, one for heat-treating the previously prepared amorphous mass and the other for melt-quenching it, have been known as the most promising means for the preparation of magnets based on rare earth elements and iron.
However, the materials obtained by these methods are in the form of thin films or strips so that they cannot be used as the magnet materials for ordinary electric circuits such as loud speakers or motors.
Furthermore, N. C. Koon et al discovered that Hc of 9 kOe was reached upon heat treated (Br=5 kG) with melt-quenched ribbons of heavy rare earth element-containing FeB base alloys to which La was added, say, (Fe.sub.0.82 B.sub.0.18).sub.0.9 Tb.sub.0.05 La.sub.0.05 (Appl. Phys. Lett. 39(10), 1981, 840-842).
In view of the fact that certain FeB base alloys are made easily amorphous, L. Kabacoff et al prepared the melt-quenched ribbons of (Fe.sub.0.8 B.sub.0.2).sub.1-x Pr.sub.x (x=0-0.3 in atomic ratio), but they showed Hc of only several Oe at room temperature (J. Appl. Phys. 53(3) 1982, 2255-2257).
The magnets obtained from such sputtered amorphous thin film or melt-quenched ribbons are thin and suffer limitations in view of size, and do not provide practical permanent magnets which can be used as such for general magnetic circuits. In other words, it is impossible to obtain bulk permanent magnets of any desired shape and size such as the prior art ferrite and rare earth-cobalt magnets. Since both the sputtered thin films and the melt-quenched ribbons are magnetically isotropic by nature, it is indeed almost impossible to obtain therefrom magnetically anisotropic permanent magnets of high performance.
Recently, the permanent magnets have increasingly been exposed to even severer circumstances--strong demagnetizing fields incidental to the thinning tendencies of magnets, strong inverted magnetic fields applied through coils or other magnets, high processing rates of current equipment, and high temperatures incidental to high loading--and, in many applications, now need possess a much higher coercive force for the stabilization of their properties. It is generally noted in this connection that the iHc of permanent magnets decreases with increases in temperature. For that reason, they will be demagnetized upon exposure to high temperatures, if their iHc is low at room temperature. However, if iHc is sufficiently high at room temperature, such demagnetization will then not substantially occur.
Ferrite or rare earth-cobalt magnets make use of additive-elements or varied composition systems to obtain a high coercive force; however, there are generally drops of saturation magnetization and (BH)max.